The complex problem of accurately classifying concealed human and other tactical targets is getting increasing attention in response to the relentless effort by the U.S. government to fight terrorism and urban conflicts around the world. Owing to a growing number of military conflicts in urban areas and modern rules of engagement that often preclude the use of wide area offensive tactics, an increased risk faces modern dismounted U.S. and allied forces. Across the globe military and law enforcement personnel are injured or killed each year because they lack the ability to track opposing threats through walls, darkness, dust, fog, debris, opaque barriers, large distances, and intentional concealment. It is well understood that a easy to use, dismounted portable, sense through the wall (STTW) technology is needed. To be fully useful, the sensor should operate in all weather conditions from a safe standoff distance.
The most promising evolving STTW platforms that at least partially address this challenge employ magnetic anomaly, electromagnetic (EM), and ultrasound sensing schemes [1-7]. The former is well suited for through wall detection of large weapon sized quantities of moving metallic objects; however, magnetic detection is easily defeated by ferric clutter near the target [8]. EM sensing is able to penetrate most building materials and openings through doors and windows and yet this technology does not penetrate metal walls. Ultrasound has the advantage of penetrating metallic materials; however this platform must be rigidly mounted against the wall and the platform employs audible thuds that are readily detected and located by the enemy.
A growing number of evolving high frequency UWB approaches detect nearly stationary human targets through thin walls by exploiting subtle time domain distortions in the signal that are caused by eye blinking or subtle movements of the thorax [1-7]. The effectiveness of these schemes is limited by the burden that the slightest motion of the target or operator (such as rocking body or hand motion) places on the receiver. Owing to a proportional introduction of phase noise that is 20 to 30 dB higher than the desired signal, any motion of the sensor or the target tends to interfere with the radar's function. Accordingly, high false alarm rates are reported unless the target is very still and the sensor is mounted to the wall or on a fixed tripod [7].
Like the present invention and numerous approaches in the literature [9-13], select disclosers in the below listed prior art improve on the foregoing by characterizing components of the returned signal that are less impacted by movement such as relative increases or decreases in signal amplitude and spectral peaks. Amplitude changes and spectral composition in the returned signal have the most utility for target discrimination in ultra-wideband sensor data because of phase jitter that is introduced by the oscillator in most conventional UWB high-speed sampling receivers. Frequency detection algorithms in particular are more widely used because they have been shown to provide the best combination of low false alarm and high probability of detection rates.
The most common threads among the present disclosure, other publications, and related prior art include the use of transceivers to transmit and receive signals, time averaging to improve signal to noise ratio (SNR), windowing functions to localize feature information, automatic target recognition methods to differentiate objects of interest from background clutter, and various types of displays to report a detected target. For example Yavuz publicly discloses the use of Hamming windows to suppress clutter in the vicinity of atmospheric targets in UWB sensor data [13]. Yavuz employs 5 fixed windows that are placed over areas of interest in the sampled signal. This approach does not scan the entire target echo and does not dynamically alter the position of the windowing function when a dielectric or other opaque occlusion distorts the time-domain signature of the target.
U.S. Pat. Nos. 6,809,520, 6,853,194, and 6,967,574, 5,552,705, disclose systems and methods that employ time domain amplitude signature recognition techniques to identify buried metallic targets. These patents disclose methods that employ pick-up coils, variable inductance antennae, and multimode impulse and frequency domain transceivers to transmit and collect electromagnetic energy; and characterization of eddy currents and time-domain amplitude decay in the returned energy to identify each target. These patents, which are intended largely for metal detection, use processor elements or stored data that serves prior reference signatures to support characterization. Similar time decay techniques are used by Toth, et al., in U.S. Pat. No. 6,480,141, which relies on the attenuation, retardation, time delay, or phase shift of microwave radiation that is reflected off of a contraband target that is hidden in a container.
Other approaches to target identification and characterization include a method disclosed in U.S. Pat. No. 6,801,155, which employs a Hidden Markov Model to characterize a sequence of Doppler radar returns; U.S. Pat. No. 5,341,142, which uses 3 independent target acquisition algorithms to acquire targets from a focal plane array seeker; U.S. Pat. No. 5,963,035, which discloses a method that employs electromagnetic induction spectroscopy and lookup tables to identify radar targets; and U.S. Pat. No. 6,335,624, which employs look up tables to characterize subsurface targets in EM ground probing radar sensor data.
U.S. Pat. No. 6,950,054 describes a method and apparatus that is intended specifically to search for concealed objects on individuals at close range by analyzing changes in the portion of the RF signal that is effected by an object being placed between the clothing and the skin. Owing to the intended spirit of the invention, the method disclosed in U.S. Pat. No. 6,950,054 employs high frequencies to search for small objects that are obscured only by clothing and lightly attenuative materials. The embodiments that are claimed therein are based largely on the characterization of a first dielectric, which is the human body, and a second dielectric, which represents the target. An audible alarm is activated whenever a concealed object that belongs to a pre-determined target group is discovered. A similar signal difference approach to support concealed object detection on a person at close range is disclosed by Chadwick in U.S. Pat. Nos. 6,243,036 and 6,342,696. These disclosures employ a method that characterizes differences between levels of polarized energy in the time domain. In one preferred embodiment of U.S. Pat. No. 6,950,054, the sampled signal is processed using a Fast Fourier Transform (FFT) to separate a first signal, which is generated by a target, from a second signal, which is generated by the human body. U.S. Pat. No. 6,967,612 uses a similar method that employs a polarized radar system to detect hidden targets at ranges of up to 200 m by comparison of energy differences between co-polarized and cross-polarized reflections from a concealed object to a reference value of similar targets.
U.S. Pat. No. 6,806,821 describes an ultra-wideband transmitter, antenna array, and hardware device that processes time-slices of returned signals into coordinates and compares the same with the coordinates of known objects in a pre-existing database. The device provides rapid and certain detection of objects of interest through walls and other opaque barriers provided that the targets are stored in a data processing device a priori. The coordinates of the processed return signals are compared to coordinates of known objects in a pre-existing database to determine whether there is a match between the return signal and a known object. The probability of detection is determined by the magnitude of the distance between the center of a target volume in an N dimensional feature space, which can be expressed by a Half Fourier Transform [14], and that of a known object of interest. A small magnitude indicates a high probability of target detection. Like most UWB sensing systems, McLemore uses a radiating pulse with a sharp rising edge whose rise time is roughly 100-500 picoseconds and whose fall-time is roughly 5 nanoseconds. U.S. Pat. No. 6,806,821 uses a transmit array that delivers an electromagnetic field on the order of 5-7 kilovolts/meter and a receiving dish that has a diameter on the order of 1 meter.
U.S. Pat. No. 6,856,272 describes a method and apparatus for early detection of threats that employs dynamic threshold generation from the received signals to determine if a potential threat exists. Time weighted average of reflected energy in individual cells in the sample space is used to generate thresholds in each corresponding analysis period. If the threshold exceeds a pre-determined value, an alarm is reported. Threats may include concealed weapons, roadside bombs, or other weapons.
U.S. Pat. No. 6,359,582 describes a weapons detector and method utilizing short pulse radar that is stepped or swept-chirped over a set of frequencies. The radar first enters a range finder mode to determine the distance between the radar and the target. The received signal is subsequently range gated and converted into an intermediate frequency (IF) signal. While this approach is suitable for narrow stepped frequency approaches, current portable wideband solutions do not adequately discern phase in a digitized sample. Owing to the strong reliance on a priori knowledge of target resonances; the method disclosed in U.S. Pat. No. 6,359,582 precludes the capacity to send data to a process or method that can be used to characterize weapons that are outside of a pre-determined resonant frequency set. Moreover, the transmitter and receiver cannot be broadband as this precludes distinguishing among resonant frequencies.
U.S. Pat. No. 6,577,269 discloses an invention that is generally intended to identify the range between a target and the sensor. The invention uses a difference signal method that employs an FFT to provide an output signal. A derivative of the FFT output signal is computed and the object is detected in response to a zero crossing of the same. The range to the object is determined by the frequency at which the zero crossing of the FFT output signal occurs. In one embodiment, the derivative is a second derivative. The invention also includes a look-up table that contains a plurality of indicators that can asses the presence of absence of an object in a specific proximity to the radar system.
Nagashima, et al., describe a device and method that employs SAR techniques to detect and identify targets that are buried underground. In U.S. Pat. No. 4,896,116 a scan is effected by moving the transmitting and receiving antennas in one direction to obtain an underground cross-sectional area that appears along the scanning direction. The area is reconstructed by synthetic aperture processing to arrive at a signal in the vicinity of the target. The resulting observation is incrementally scanned and analyzed by a zero-crossing window over each segment of the signal. Each separated waveform is converted into a frequency region by a FFT to determine its spectral distribution. The frequency, amplitude, and DC component of the strongest spectral peak are used to compute a DC component ratio. If the DC component ratio falls within a certain range, the echo is treated as a target, otherwise it is returned as a spurious echo.
This disclosure supersedes prior art in part by specifying a system that reliably detects, characterizes, and intuitively displays any plurality of mobile and stationary objects of interest that are found in ultra-wideband sensor data. This invention addresses the foregoing in part by employing wide sensor bandwidth and feature vectors that do not rely on incremental operating frequencies or the differences between individual dielectrics to discern objects of interest from background clutter. Automatic target recognition (ATR) performances is further improved in this invention by using both dielectric and conductivity information to construct each feature vector. The disclosed invention also improves prior art by eliminating range gating methods that limit the portion of the returned signal that is analyzed for signal anomalies to a narrow region that is near a specific background clutter object such as a human subject. While analyzing the complete radar return, this invention uses amplitude peaks in the time-domain signature to dynamically position the windowing function as it is slid through the signal. Look-up tables that impede the capacity to identify targets without a priori knowledge and subtle motion detection schemes that place an excessive burden on the receiver are also obviated by this invention.